Valves are essential parts of the pressure pipes used in petrochemical plants, oil fields, metallurgical plants and thermo-electric plants. In actual applications of the valves in the prior art, the medium pressure must be overcome to allow the opening/closing part to open and close the valves. When the valves are used in high temperature and high pressure pipes with large diameters, the medium pressure may bring large operating force to the valves and the sealing surfaces of the valves are easy to be damaged to cause leakage.
Pressure self-balancing valves follow the pressure self-balancing principle. The medium pressure on the opening/closing part can be self-balanced, therefore influence on operating performance, sealing quality and sealing lifespan of the valves is eliminated. The pressure self-balancing valves have become the target for the valve products in the prior art to be upgraded. The key technology is the sealing of the pressure self-balancing structure. The pressure self-balancing structure in the prior art generally adopts O or Y-shaped seal rings to provide sealing. Since self-sealing is achieved based on the medium pressure against the seal rings, the sealing degree is irrelevant with either the medium pressure or the sealing pre-tightening force. The sealing degree will not change when the sealing pre-tightening force drops after the seal rings wear; therefore durable and reliable sealing quality and sealing lifespan are achieved. Because the O or Y-shaped seal rings must be made of elastic materials with high density, the elastic materials are rubber or plastic that cannot stand high temperature. Due to the restriction of those elastic materials, the applicable temperature of the pressure self-balancing stop valves is still restricted below 200° C. and the stop valves cannot be used in high temperature and high pressure pipes that are large in applications and quantity.
Studies show that some low density sealing materials are high temperature resistant or corrosion resistant. For example, flexible graphite is a type of loose and high temperature resistant material with low density, and under high temperature, high pressure and radiation conditions, it is unlike to dissociate, deform or age. As it has stable chemical property, it is often used as the sealing packing for high temperature rods. As flexible graphite is not high density material, it cannot be used in O or Y-shaped seal rings. In the patent CN 2637835Y, applied in 2003, the inventor used high temperature resistant flexible graphite as the sealing material for the self-balancing stop valves. It solves the problem that the sealing material cannot stand high temperature. In that patent, the seal rings are made of high temperature resistant flexible graphite and press rings are applied to press the seal rings to solve the low density problem exist in high temperature resistant sealing materials, thus the excircle of the valve disc and the valve body can be sealed properly. However, the sealing structure in such structure does not have pressure self-sealing property; the sealing degree largely depended on the sealing pre-tightening force once the press rings tightly press the seal rings. In actual applications, the sealing degree will drop when the seal rings are worn by the valve disc. Consequently the sealing lifespan in that patent fails to meet the actual requirements.
In order to improve the applicability of the flexible graphite, Mr. Liu Wang disclosed a new structure stop valve with the following technical characteristics (see Journal of the Chemical Fertilizer Industry: lines 11-24 on the right column of page 39, vol. 2, 1989, schematic diagram of new structure of stop valve): a packing cover, packing and washers are disposed from top to bottom inside the valve. A handle is tightened to allow the packing cover and the washer to squeeze the packing together. The contact surface between the packing cover and the packing is a spherical surface to improve the sealing degree. The spherical surface can also compensate the wear at the upper side of the packing at any time. In that article, the packing cover is used to impose the pressing force on the packing to achieve the sealing on the valve head rod by the packing. The spherical surface of the contact surface between the packing cover and the packing is used to enhance the pressing force on the valve head rod by the packing so as to further strengthen the sealing on the valve head rod by the packing incircle. However, in actual applications, the sealing on the valve head depends on both the sealing on the valve head rod by the packing incircle and the sealing between the packing excircle and the packing box. But after the packing incircle is worn by the valve head rod, the sealing pre-tightening force of the packing incircle/excircle will drop accordingly. When the sealing pre-tightening force is lower than the medium pressure, the leakage will occur between the packing excircle and the packing box, where the pre-tightening force is relatively low. The leakage will occur between the excircle of the packing cover and the packing box. In the article, it mentioned that the packing needs not to be pressed for one or two months. Therefore it can be understood that the sealing lifespan of the stop valve is low and the packing must be pressed many times or in time to maintain the sealing on the valve head. The technical solution disclosed in that article is not applicable to the sealing environment of the pressure self-balancing valve.